During the fabrication of various parts, such as aircraft parts or parts for other applications, a hot press, such as a hot forming press or a superplastic forming press, may be utilized to heat and form the parts. In order to shape the parts, a hot press may include a die defining a mold surface again which a workpiece, such as a sheet of titanium, is pressed. During superplastic forming operations, such as through the application of an elevated temperature and a pressure differential between opposite sides of the workpiece, the workpiece may be formed so as to have the shape defined by the mold surface. In order to form the part so as to have the desired shape, the die should advantageously maintain the integrity of the mold surface throughout one or more superplastic forming operations.
Some dies are formed of metal alloys that withstand the repeated high temperature superplastic forming cycles, such as by withstanding the repeated exposure to temperatures up to 1650° F. utilized in a superplastic forming process. While a die that is formed of these metal alloys has a relatively long life as measured in terms of the number of superplastic forming cycles, dies formed of these metal alloys are quite expensive. As an alternative to the expensive metal dies, ceramic dies are sometimes utilized in conjunction with superplastic forming operations. Ceramic dies are much more economical, but typically have a relatively short life. For example, ceramic dies generally withstand only 10 or fewer superplastic forming cycles and, in some instances, withstand no more than two superplastic forming cycles prior to failure, thereby requiring the ceramic dies to be repeatedly replaced.
In regards to the failure of a ceramic die, ceramic dies are formed of materials that are somewhat brittle and have a relatively low tensile strength. Although ceramic dies may be reinforced with fused quartz rods to add a compressive stress field in regions near the rods, the fused quartz rods can only be placed at certain locations and cannot be spaced throughout the entirety of the ceramic die such that their effect is somewhat limited. As such, ceramic dies may fail when subjected to the high temperature forming cycles and to the pressures exerted during superplastic forming operations. In this regard, ceramic dies may fail by the formation and propagation of cracks through the mold surface defined by the ceramic dies, such as near the interior corners of the mold surface. Further, as a result of the relatively low tensile strength of the ceramic material that forms the dies, any sticking of the ceramic material to the part being formed results in portions of the mold surface of the ceramic die flaking off, thereby damaging both the ceramic die and the part being formed.
In addition to the relatively short life of ceramic dies, the interruption that is created upon the failure of a die, such as a ceramic die, during a superplastic forming operation is also costly, both financially and in terms of down time. In this regard, the failure of a die, such as a ceramic die, during a superplastic forming operation will cause a significant delay in the superplastic forming process in order to cool the hot press, remove the ceramic die that has failed, insert another ceramic die within the hot press and then reheat the hot press. This process of replacing a ceramic die that has failed may delay the superplastic forming process by several hours and incur significant costs. In addition, the relatively short life of ceramic dies generally requires that one or more additional ceramic dies be maintained as spare parts so as to facilitate such repairs in the event of the failure of the ceramic die currently in use.